This web site has some serious issues to be honest.
I use Chrome and on laptop at home it consumes lots of CPU and entering answers is slow, as OP said.
At work on a different laptop it also has issues. I have a very powerful laptop at work and I have approx 12 chrome tabs on the go and the one that falls over in the background is this one. Also same issue with slowness in entering answers.
However on iPad pro 128GB with Safari it behaves.